1. Field of the Invention
The present invention relates generally to convergence measurements in color cathode ray tube (CRT) display assemblies of the type using a multiplicity of electron beams for illuminating a phosphorescent screen, and more particularly to display assemblies of the well known shadow mask type.
2. Description of the Prior Art
It is well known to those skilled in the color CRT art that separate beams of energy are generated, usually from three separate guns, which beams are normally focused on a screen or mask spaced from the interior surface of the CRT viewing surface. For example, in a shadow mask structure the screen is comprised of a multiplicity of minute openings through which the beam triad passes and then diverges to energize corresponding dot triads of red, green and blue phosphors, resulting in red, green, and blue light emissions from the face of the CRT. Other phosphor configurations, such as oblong regions or even parallel stripes may also be employed. The problem of misconvergence arises when the separate beams of energy from the red, green and blue color guns do not impinge at the same point of the viewing surface. It will be appreciated that manufacturing tolerances in the guns, mounts, tube neck, and mask and faceplate geometry, as well as inherent nonuniformity or nonlinearity of the magnetic fields generated by the beam deflection coils will tend to deflect a particular color beam from its intended path. In order to assure convergence over the entire viewing surface, each of the several beams of energy must intersect precisely at a predetermined hole of the mask. The term convergence is defined as making each of the several beams of energy all coincide at the same point as that point progresses in the scan across the viewing surface.
Related to convergence is the concept of linewidth. As a phosphorescent array is sequentially scanned across a transverse axis on the face of the viewing screen, the apparent brightness of the line in a direction orthogonal to the scanned axis varies, being most intense at the center of the phosphorescent region. Linewidth may be conveniently defined as the distance across the phosphorescent region, such as a dot, at which the brightness is observed to diminish to 50% of its peak intensity.
In practice, it has not been possible to obtain perfect convergence, such as is required for flight instrument display systems, which are viewed at close range, purely by mechanical means or deflection coil design. In consequence, a multiplicity of schemes has been developed for applying an electronic compensation, such as is described in U.S. Pat. No. 4,385,259, issued May 24, 1983, to Carl L. Chase, et al and assigned to the assignee of the present invention. In that scheme, analog coarse information, which is a function of the longitudinal and vertical positions of the beams, and fine compensation, provided by digital programable-read-only-memories, were summed together and applied to the convergence correction coils of the CRT.
In applying convergence correction schemes, such as that referenced above, it is necessary to perform a calibration wherein the linewidth can be measured and the degree of misconvergence determined so that the display may be optimized and convergence errors minimized. One current technique for measuring linewidth, for example, is to provide a stationary light sensor responsive to a display line which is scanned on the face of the screen. The sensing area of the light sensor is made extremely minute, much smaller than the displayed linewidth, and therefore may be centered on a phosphor area of the desired color. The scan is accomplished by applying a repetitive voltage waveform to one axis of the deflection yoke. The voltage across the yoke is applied together with an output from the light sensor and plotted on an x-y recorder. The x-axis, for example, may be calibrated to correlate with the scanned distance of the displayed line and the detected light intensity applied to the y axis. The linewidth then can be calculated by conventional techniques (i.e. as by defining the linewidth to be the width at the one-half peak brightness points).
Convergence may be measured by moving a light sensor a known distance to another phosphor area and plotting a second linewidth of another color. The difference between peaks of the plotted responses is a measure of the degree of misconvergence. If the beams are not exactly aligned on the mask apertures, the difference between peaks will not correspond to the distance the light sensor has been moved. Perfect convergence occurs when the peaks of the three color beams are superimposed or displaced a distance corresponding to the mask aperture spacing, depending on the orientation of the mask with respect to the light sensor.
Another approach for adjusting a convergence apparatus is described in U.S. Pat. No. 4,291,256, issued to R. L. Garvin on Sept. 22, 1981, wherein a photodiode detector is focused on a single spot on the display face and the display unit is equipped to independently disable, position and intensify each beam. However, this system does not provide for quantitative measurements of convergence.
The above techniques work where the associated electronic convergence correction apparatus can be disabled. However, when convergence measurements must be made on a display instrument which has complex dynamic electronic controls for convergence which must remain in synchronism with the deflection commands, auxiliary interface circuitry must be developed to assure no interaction between the dynamic correction functions and the test circuitry. Moreover, these techniques require that the instrument be opened up and the yoke leads disconnected and brought out to the test and control apparatus. By interfacing the electronics inside the instrument to the external circuitry of the test apparatus, there is always some risk of operating the CRT in an abnormal manner and hence invalidating any measurements. For example, the increased lead length may result in noise and jitter on the sweep signal. Further, the interfacing electronics must be adapted or reconstructed to conform to each type of display unit to be tested and therefore is not readily interchangeable between different models of display units.